Field of Invention
The present invention relates to a field of control technique for well bore pressure, and more particularly to a method for controlling well bore pressure based on model prediction control theory and systems theory, which is capable of ensuring that the pressure traverse of the well bottom or the well bore controlled thereby is in a safety window, and that wellhead pressure controlled thereby is safe for the well bore.
Description of Related Arts
In recent years, with the increasingly development of petroleum and natural gas exploration and exploitation, well drilling is increasingly processed in various complicated structure areas. The conventional OBD pressure control technique is not capable of meeting production requirements such as well drilling in complicated structure areas, narrow density window security drilling, drilling in H2S bearing layer, diameter-shrinkage bit block caused by high density mud leakage and well-control risk caused by high density mud leakage. Since the OBD pressure control technique is still a type of conventional manual rough pressure control, which achieves the object of controlling the well bottom pressure depending on experiences of the onsite operators, wherein a relative steady state often can not be achieved in the well by regulating the throttle valve repeatedly, and furthermore, the well bottom pressure has a wide fluctuation which can not be controlled in a small range to have an approximately constant well bottom pressure. However, adopting fine pressure control technique is capable of processing well drilling in complicated structure areas, such as narrow density window drilling, and decreasing 80% of the problems encountered in the conventional well drilling technique.
Because the well is a fuzzy system with large quantity of uncertainties therein, the conventional wellhead constant pressure control measure results in a failure of well bore pressure fine control or even causes accidents. Particularly in the condition of overflow, the wellhead, the casing pressure increases opening degree of the throttle valve, which is reflected in the wellhead, but actually the overflow causes that the bottom fluid further enters the well bore, which decreases the well bottom pressure. In addition, during the process of drilling, fluctuation of the well bottom pressure and the well bore pressure is required to be smaller and smaller, and a failure of pressure control is easily caused even by a slight mistake, so that complicated accidents such as well overflow are caused.
A large quantity of the conventional pressure control methods are focused on well bore flow. However, after searching, a set of pressure calculating prediction control method which is capable of ensuring a safe pressure control for the well bore at any time has not been disclosed yet. If the problem can not be satisfactorily solved, the popularization and application effects of underbalanced drilling technique (UBD) and managed pressure drilling technique (MPD) are directly influenced; risk of well drilling control is increased; and cost of well drilling is high, so that a large quantity of oil fields which are supposed to be developed earlier can not be developed in time.
Yang Xiongwen, Zhou Yingcao, Fang Shiliang and Liu Wei disclose a periodical literature with a title: Design and laboratory test of hierarchical intelligent control system for managed pressure drilling on a Journal Petroleum Drilling Techniques, Vol. 39 No. 4, July 2011, wherein an MPD multi-level hierarchical control strategy is disclosed, but technical problems are still not solved as follows.
1. The control objective of is to control wellhead pressure. Though an objective mentioned in the literature is to control the well bottom pressure, in the block diagrams 2, 3 and 4 and descriptions thereof, the objective is based on controlling the wellhead pressure. The wellhead pressure control is only a small part of the practical well bore pressure control, which is equivalent to manipulating the conventional manually operated throttle by people, so as to ensure that the wellhead pressure is equal to a set value. However, it is still an unresolved issue of how to control the well bottom pressure by controlling the wellhead pressure, i.e., how to control the well bottom pressure to a set value.
2. In the calculation, it is difficult to obtain exactly the overflow discharge. The overflow discharge monitored at the wellhead is variations of the overflow while reaching the wellhead. Depending on the overflow discharge monitored thereof for calculating and controlling is already too late, and thus an object of precision control can not be reached.